Small-sized crystal oscillator devices are most often used for making reference frequency generators in particular for electronic equipment, in numerous fields such as horology, information technology, telecommunications, GPS, and the medical field.
Small-sized crystal oscillator devices are often (but not necessarily) SMDs (Surface Mounted Devices). A small-sized surface-mount crystal oscillator device is known, which comprises a container for surface mounting, a piezoelectric resonator element (the crystal), and an integrated circuit chip (IC chip) connected to the piezoelectric resonator element so as to form an oscillation circuit. The IC chip and the piezoelectric resonator element are both arranged inside a hermetically sealed cavity of the container.
The resonator frequency output signal from a crystal oscillator device will generally change as the temperature of the oscillator changes. Various methods are known to neutralize this effect caused by changes in the ambient temperature. For example, crystal oscillator devices are known that comprise an oven for heating the oscillation circuitry and/or the piezoelectric resonator element.
Oven Controlled Crystal Oscillators (OCXO) usually contain a heater and a temperature sensor along with a heating control circuit to control the heater. The heating control circuit controls the power supplied to the heater as a function of ambient temperature. The amount of power supplied changes with ambient temperature, in such a way as to hold the crystal and other critical circuitry at a predetermined constant temperature. This predetermined temperature is usually chosen to be about 10 degrees above the highest expected ambient temperature.
The resonator frequency of a piezoelectric resonator element can generally be approximated by one of either a square or a cubic function of temperature. There is usually at least one stationary point on the curve where the slope of frequency versus temperature is zero. The piezoelectric resonator element should be chosen so that a stationary point coincides with the desired constant temperature of the oven. In this way, inevitable temperature cycling around the predetermined oven temperature will have only a marginal effect on the frequency stability.
In an OCXO, the piezoelectric resonator element is usually enclosed in a case and the entire case is covered by a heater. Such an oven controlled oscillator device has excellent temperature characteristics. However, it also has the disadvantage of large power consumption and long warm-up time. Patent document U.S. Pat. No. 5,917,272 discloses a crystal oscillator device with reduced power consumption. The oscillator device comprises a piezoelectric resonator element mounted in a thermal conductive manner over a heat conductive substrate by means of highly heat conducting support clips. As the support clips are electrically conductive, they also serve to electrically connect the piezoelectric resonator to conducting paths on the surface of the substrate. The heat conductive substrate also carries a heating device, a control circuit and a temperature sensor. The substrate, itself, is mounted in a heat insulating manner inside an airtight package, by means of thermal insulating posts. Electrical leads are further arranged inside the package to connect the substrate to connecting pads which in turn are connected to the exterior. The electrical leads are made of very fine wires in order to limit heat conduction. As the substrate carrying the heating device and the piezoelectric resonator element is reasonably well insulated from the walls of the airtight package, heat dissipation is greatly reduced.
However, such prior art crystal oscillator devices have some problems. Indeed, when the ambient temperature changes, the temperature of the outer surfaces of the device also changes. As the device is at least partially packaged in heat conductive material, the ambient temperature change propagates inwards. As the piezoelectric element lies directly under the cover, a certain amount of heat is susceptible to radiate from the cover to the crystal element or vice versa. Furthermore, the oscillation circuitry is provided outside the package. Therefore, ambient temperature changes can be transmitted to the piezoelectric resonator unit, by heat conduction, through the oscillation circuitry, the electrical leads, and finally the support clips.
Ambient temperature changes must first reach the temperature sensor mounted on the substrate before the control circuit can activate the heater. Since the piezoelectric resonator unit is spaced from the temperature sensor, the rate of temperature change in the two units is different. This can lead to a delay before the temperature control circuit reacts. Furthermore, once the temperature control has reacted, it takes time for the heat produced by the heater to reach the piezoelectric resonator unit. It follows that there is a risk that the temperature of the piezoelectric resonator unit will deviate substantially from the set-point temperature of the device. Therefore, a really stable oscillation can not be guaranteed.